Human CDI proteins are a family of norimajor histocompatibility complex (MHC) encoded transmembrane glycoproteins expressed in association with 02-microglobulin on the surface of antigen-presenting cells (APCs). Unlike the well known M[HC class I and class 11 proteins that present peptide antigens to T cells, the human group I CD1 proteins (CD1a, CD1b andCDlc) mediate specific T cell recognition of bacterial lipid and glycolipid antigens. Previous studies of mycobacteria specific T cells have identified two classes of CDI-restricted lipid antigens. These are the free mycolic acids, a family of cc-branched, 0-hydroxy long chain fatty acids, and the phosphatidylinositol-containing glycolipids including lipoarabinomannan (LAM) andthe phosphatidylinositol mannosides (PlMs). Mycohacterium leprae infection has recently revealed evidence for a third class of CDI restricted lipid antigens. The CDI proteins play a central role in the specific T cell recognition of foreign lipid antigens, but the molecular mechanisms underlying lipid antigen presentation are not known. During this past year, we have identified a novel CDI-restricted glycolipid antigen, glucose monomycolate (GM[M), which is allowing a systematic analysis of the structural features that determine its recognition by T cells. Analogues of GNM that differed substantially in their acyl chain lengths and other chemical features of the lipid moiety were recognized by T cells. In contrast, T cells demonstrated fine specificity for the carbohydrate portion ?of mycolyl glycolipids, even discriminating among carbohydrate isomers differing only in the orientation of a single hydroxyl group (e.g., only glucose and not mannose or galactose). In combination with recent studies of the crystal structure of CD1, these results provide strong support for a molecular model of antigen presentation in which the acyl chains of the antigen bind relatively nonspecifically within the deep, hydrophobic pocket of the CD1 protein, resulting in presentation of the hydrophilic elements of antigens for highly specific interactions with the T cell receptor.